Field of the Invention
The present invention relates to a zoom lens used in a TV camera, a video camera and a digital camera, and used particularly for the purpose of monitoring, and also relates to an imaging device using the zoom lens.
Description of the Related Art
There are wide-ranging demands from users for TV cameras and video cameras. Especially, high image quality and compact size are always demanded by the users, and much weight is given thereto. Therefore, high performance and compact size are both required also for a zoom lens used in the TV camera or video camera. There are also demands for a zoom lens having as large a variable magnification ratio as possible. For the purpose of relatively long-distance monitoring, a relatively compact zoom lens having a variable magnification ratio exceeding 25 times and close to 30 times is supported by the market as one of the options.
Various types of such a zoom lens have been studied for use in a TV camera and the like. For example, as a zoom lens suitable for high variable magnification, there has been known one including: a first lens group having a positive focal length; a second lens group having a negative focal length; a third lens group having a negative focal length; and a fourth lens group having a positive focal length, the first to fourth lens groups being arranged in this order starting from the object side to the image side, in which, in magnification change, the second and third lens groups are moved so that intervals between the first and second lens groups, between the second and third lens groups and between the third and fourth lens groups are changed (see, e.g., Japanese Patent Application Publication No. 2010-175956).
As this type of zoom lens, there have been proposed one having a diffractive optical element provided in the first lens group (see, e.g., Japanese Patent No. 4928297 (Japanese Patent Application Publication No. 2008-197534)), one using extra-low dispersion glass in the first and fourth lens groups (see, e.g., Japanese Patent No. 5006076 (Japanese Patent Application Publication No. 2008-241884)), and the like.
In many zoom lenses with such a configuration, the second lens group functions as a variator for magnification change and the third lens group functions as a compensator for correcting a shift in image plane position due to magnification change. However, there is also a zoom lens having the third lens group also take charge of part of a magnification change operation for achieving both compact size and high variable magnification. Meanwhile, the fourth lens group takes charge of a main imaging operation as a master group, and the configuration of the fourth lens group is very important for favorably correcting various aberrations and ensuring high imaging performance.
Particularly, a TV camera for monitoring sometimes performs imaging with sensitivity up to near-infrared range having a wavelength of about 900 nm or less. For example, during the day with enough light, an accurate color image is obtained using only visible light while blocking near-infrared light. In a bad weather or in the twilight and dawn, an amount of light is earned by transmitting all the light from the visible range to near-infrared range. During the night, infrared having a wavelength of about 850 nm is projected and irradiated while blocking the visible light.
Thus, for a zoom lens used as a photographing lens, it is required to correct chromatic aberrations not only in the visible range but also in the near-infrared range. This is because, when chromatic aberrations up to the near-infrared range are not corrected, there arises a need for refocusing in switching between visible light and near-infrared light. Moreover, sufficient resolving power cannot be obtained when the zoom lens is used in a manner to transmit all the light from the visible range to near-infrared range.
Japanese Patent Application Publication No. 2010-175956 aims at favorable aberration correction by devising the configuration of the fourth lens group. However, in the zoom lens disclosed in Japanese Patent Application Publication No. 2010-175956, while good imaging performance is achieved in the visible range, aberrations up to the near-infrared range are not corrected. Moreover, the variable magnification ratio tops out at about 10 times.
On the other hand, the zoom lens disclosed in Japanese Patent No. 4928297 (Japanese Patent Application Publication No. 2008-197534) uses the diffractive optical element in the first lens group and performs chromatic aberration correction using negative dispersion of the diffractive optical element. However, again, only the visible range is considered and no aberrations up to the near-infrared range are corrected. Moreover, as for a zoom lens having a variable magnification ratio exceeding 25 times, a telephoto ratio (a ratio of the overall length of the lens to the focal length) at a telephoto end is as large as around 1.0. Thus, sufficient reduction in size is not achieved.
Meanwhile, in the zoom lens disclosed in Japanese Patent No. 5006076 (Japanese Patent Application Publication No. 2008-241884), aberrations up to the near-infrared range are corrected using the extra-low dispersion glass in the first lens group with a large axial marginal ray height on the telephoto side as well as in the fourth lens group with a large axial marginal ray height on the wide angle side. However, the variable magnification ratio is about 22 times, which is smaller than 25 times.